Typically earthmoving machines, such as for example backhoe loaders and excavators, but in general many vehicles for industrial and agricultural use, use a transmission to provide the motive power required to move them.
The hydraulic circuits used in these transmissions typically comprise a pump to raise the oil to the working pressure and deliver it to a lubrication circuit and the vehicle transmission components. The pump is driven by an internal combustion engine which is caused to work at variable speeds and powers depending upon the pressure/power required to move the vehicle.
The oil pressure in the hydraulic circuit is however maintained above a lower limit value because a certain throughput is required in order to lubricate the transmission even when the vehicle is stopped. In this type of vehicle provision is also made for a so-called power take-off, that is to say an output shaft, also known as a PTO shaft, coupled to the internal combustion engine, which drives the vehicle's auxiliary pump and which is used to transmit power from the internal combustion engine to the moving arm of the excavator, in the case of an earthmoving machine, or in general to other working components.
For example, again in the case of vehicles of this type, the power take-off is connected to the pump of another hydraulic circuit which brings about movement of the arm and other tools.
In general, two different working situations for the vehicle can be envisaged—a first situation in which the vehicle is moving and power may be requested for the vehicle's hydraulic system, and a second situation in which the power to the vehicle's hydraulic circuit is provided when it is stopped. In the first case power has to be provided to both the hydraulic circuits, while in the second case the working components have to be fed and only the minimum hydraulic flow for lubrication is provided to the transmission.
However in the latter case both the power provided to the working components and the pressure provided to the transmission depend on the rotation speed of the internal combustion engine and therefore in known vehicles it is not possible to provide high power to the working components and limit the pressure to the hydraulic circuit of the transmission to the minimum values necessary to ensure adequate lubrication at the same time.
In fact, in the example of a backhoe loader or an excavator, the transmission is not required to operate when it is engaged purely in excavation work with the vehicle stopped.
However the internal combustion engine is nevertheless made to work at a high rotation speed to deliver power to the hydraulic circuit of the working components, the pump for this circuit being driven by the power take-off shaft.
In this way, when purely excavation work is being carried out the hydraulic system of the transmission is also inevitably maintained at a high pressure, even though this is not required.
As the pump controlling the transmission has a fixed cylinder capacity there will be an expenditure of power to maintain pressure in the transmission's hydraulic system which will depend on the throughput delivered by the pump and the main pressure in the hydraulic circuit of the transmission.
This gives rise to consumption of power P from the internal combustion engine which is dissipated without being utilized, with a consequent inefficiency for the vehicle.
This is even more true when it is borne in mind that these vehicles are mainly used for purely excavation work during their lives.
It will therefore be desirable to avoid such energy wastage.
The technical problem which underlies this invention is therefore that of providing an industrial vehicle which makes it possible to overcome the abovementioned disadvantages with reference to the known art.